Reorganize "Contributing" documentation to be more accessible to new contributors (#573)

* Reorganize "Contributing" documentation to be more accessible to new contributors

The following changes were done:

- move "How to contribute" first, and "Set up a dev environment" as the
  second section.  This way, a new contributor has access to the most
  general information first, instead of the very specific "dev
  environment" documentation.

- reduce "How to contribute" / "As a developer" by moving the part about
  tests to a new "Contributing as a developer" section (see below).  This
  way, all types of contributions get roughly the same amount of text in
  the first "How to contribute" section.

- add a new "Contributing as a developer" section, which lists items that
  are useful when preparing a code contribution (running tests, formatting
  code, create database migration).  These items were moved either from
  "How to contribute" or from "Set up a dev environment".

* Add brief documentation about updating and adding tests

Co-authored-by: Baptiste Jonglez <git@bitsofnetworks.org>

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